This invention relates to methods of making the body of a hydraulic master cylinder, particularly of the type used on automobiles, motorcycles and other vehicles as means for actuating a hydraulic brake system or a clutch assembly.
In general, the body of a hydraulic master cylinder is complicated in shape and not uniform in wall thickness and, even when formed by die-casting, such cylinder body tends to include localized shrinkage cavities. Should shrinkage cavities be formed in the bore wall section of the cylinder body, the piston cup, fitted to slide over the bore wall surface, must be injured by any sharp edge of the cavities and its sealing capacity impaired. Also, formation of such cavities results in loss of oil-tightness of the bore wall itself, possibly allowing hydraulic oil to leak through the cavities when the oil pressure in the bore is raised. In an attempt to overcome such inconveniences, it has previously been proposed to incorporate a length of steel pipe, which is highly impervious to hydraulic oil, in the casting of a master cylinder body as an insert in order to define a cylinder bore therein as the hollow interior of the steel pipe. However, as the cylinder body is usually cast from an aluminum alloy, which is quite different in nature from steel, a more or less clearance inevitably forms at the interface between the cylinder body and the steel pipe insert on account of the difference in coefficient of thermal expansion and the limited adhesion between the two materials. In use of such cylinder body the clearance tends to cause oil leakage, communicating with oil passages, such as vent and replenishing ports interconnecting the cylinder bore and an oil reservoir formed integral with the cylinder body.